1. Field of the Invention
The present invention relates to a printed circuit board unit comprising a printed circuit board and an electronic component such as a chip mounted on the surface of the printed circuit board through solders.
2. Description of the Prior Art
Soldering is often employed to mount electronic components such as semiconductor devices or bare chips on a printed circuit board in a multi-chip module (MCM) or a semiconductor package. Soldering is also employed to mount electronic components such as the MCMs on a printed circuit board of a larger size, namely, a motherboard. When the individual bare chip or MCM is to be exchanged in the MCM or motherboard, solders must be broken or removed to release bonding between input/output terminals on the bare chip or the MCM and input/output pads arranged on the printed circuit board. For example, an exchanging operation is designed to detach the older bare chip or MCM from the printed circuit board when the solders are subjected to heat of temperature higher than the melting point of the solders. A new bare chip or MCM is thereafter mounted on the printed circuit board by soldering input/output terminals of the new bare chip or MCM to the input/output pads which have previously received the input/output terminals of the old bare chip or MCM.
It is well known that part of the old solder still remains on the input/output pad on the printed circuit board even after the old bare chip or MCM is detached in the above-described exchanging operation. When a new bare chip or MCM is mounted on the printed circuit board, a new solder of a predetermined amount is added to the old solder remaining on the input/output pad. An excessive solder may induce a short between the adjacent input/output terminals.
In particular, the respective input/output pads are not expected to keep solders of a uniform amount remaining thereon. When a bare chip or MCM employs a plurality of input/output terminals such as a ball grid array (BGA) and a pin grid array (PGA), the bare chip or MCM may suffer from the solders of a varied height on the input/output pads. The input/output terminals of the new bare chip or MCM may in part fail to touch the surface of the input/output pads on the printed circuit board. No electric connection can thus be achieved partly.
In view of the above inconvenience, the solders remaining on the input/output pads should be wiped out before a new bare chip or MCM is mounted. For example, the printed circuit board along with the remaining bare chips or MCMs is subjected to another heat so as to melt the solders remaining on the input/output pads. Heat is sometimes repeatedly applied to the printed circuit board more than twice until the solders are completely wiped out. Repeated application of heat is supposed to damage and shorten the life of the printed circuit board, and bare chips or MCMs remaining on the printed circuit board. It sometimes deteriorates the wetness of the input/output pads to solders.